The present invention relates to a multilayer optical disk reproduction system for reproducing multilayer optical disks having two or more signal recorded layers, and more specifically to improvements in a focus control device and method for selectively performing focus servo on the signal recorded layers.
As is well known, a type of optical disk, called a DVD (Digital Video Disk), has been developed and put into practical use in recent years. This DVD, which is 12 cm in diameter as is the music compact disk (CD), can be recorded on its one side with more than five gigabytes of digital data.
For this reason, it is expected that the DVD will be widely used in various fields as a large-volume storage medium. With the DVD, a further increase in recording capacity can be achieved by sticking a plurality of disks each with a signal recording layer together into multilayer structure.
In an optical disk playback system adapted to reproduce such a multilayer DVD, focus servo is performed on the objective lens so that a laser beam is focused onto a signal recorded layer on which, of multiple signal recorded layers, a reproduction request is made, thereby reading data recorded on that layer.
Suppose here that, when focus servo is being performed on a specific signal recorded layer, a request is made for playing back another signal recorded layer. In such a case, in order to perform focus servo on the target signal recorded layer, it is required that the multilayer optical disk reproduction system be equipped with what is called a layer jump function to automatically control the position of the objective lens.
Also, suppose that, when none of the signal recorded layers is played back, that is, when the objective lens is placed in its initial position, a request is made for playing back a desired signal recorded layer. In order to perform focus servo on that signal recorded layer, it is required for the multilayer optical disk reproduction system to be equipped with a focus search function that allows the position of the objective lens to be controlled automatically.